Laying SWA cable to a new detached garage

[aaronjb]

I've been reading past threads and so on here to try and figure out the best way to achieve this, and from what I can gather that's:

1) Install conduit now complete with draw rope
2) Have an Electrician who is a member of the 'competent persons' scheme install the SWA, garage CU and so on.. essentially making this no longer a DIY project at all.

Otherwise I have to notify building control and so on myself, whereas having an electrician do it, they're notified after the fact by him?

So if that's the case, two questions:

1) My house is entirely concrete floored, so taking the cable down through floorboards and out under the footings (as in the document linked on the Wiki) is nigh on impossible without digging out a large area of the houses concrete pad. Can the SWA exit the building through the wall and/or can the SWA rise up the external wall and terminate in an external weatherproof junction box? (Provided it is suitably glanded and earthed) Obviously terminating the SWA externally is going to be cheaper - on the order of hundreds or thousands of pounds, I imagine, given the floor digging required.

2) Should the conduit for the garage emerge inside or outside the planned garage wall? I presume 'inside'..

3) Oh, there'll also be an electric supply to a pit in the garage - I'm presuming I should also lay conduit from the same area as the other (i.e. under wherever the CU will be) over to the hole the pit will be going in, complete with warning tape and so on - this conduit will be under the concrete slab in entirety..


Am I right, wrong, or miles away?

 

[flameport]

1 - Bring cable in above the floor level. No need for any junction box, just take the cable through the wall and on to the consumer unit or whatever it connects to. Just make sure the hole is at a suitable angle and dres the bricks/blocks appropriately to avoid damage to the cable.

2 - Either way. If outside, then do the same as for 1.

3 - A pit in the garage will be a very expensive can of worms, don't go there.

 

[aaronjb]

Thanks for that - bringing the cable in above floor level makes it all so much easier and, no doubt, cheaper if a spark is doing it (which it probably will be - I'd love to do it myself, aside from hooking it up to the house feed, but Part P put paid to that!)

As for the pit, any reason you think it'll be a can o' worms? Other than making sure it doesn't slowly fill itself with water, of course..

 

[ban-all-sheds]

3 - A pit in the garage will be a very expensive can of worms, don't go there.

How so?

My dad had one in his garage - jolly useful.

 

[SgtTrojan]

Is that where you burn the rogue electricians sheds? Hehe

 

[flameport]

Pits in garages
- will fill with water if not properly sealed, and properly sealing them is virtually impossible
- will fill with leaking petrol / exhaust fumes / other gases
- are a potentially explosive environment, which makes the installation of lights and other electrical equipment far more expensive, and far beyond the scope of most electricians, never mind DIY.
- are a hazard as people / cars / pets / other can fall into them

 

[aaronjb]

Ahh just those reasons, then

Properly sealing them is easy - just buy a fibreglass pit liner, though it helps if the water table is low (which it is here, living on the top of a slope)
In 8 years of using a friends (fibreglass) pit, none of us have blown each other up despite plenty of fuel/oil/coolant/gearbox oil (ugh!) ending up in them/on me (always on me when it's gearbox oil, never could figure that one out!)
And you don't fall in if you build a lid for it although we did once find a field mouse in his (and plenty of spiders, beetles and other creepy crawlies)

I should probably add that his was even signed off by building regs (since it's in an attached garage >30m2)..

 

[Adam_151]

- are a potentially explosive environment, which makes the installation of lights and other electrical equipment far more expensive, and far beyond the scope of most electricians, never mind DIY.

Having never really got involved with ATEX classified areas meself, can you tell me what the requirements for lighting in an inspection pit in a domestic garage would normally include, I'm just having difficultly visulaising how having say a GLS bulkhead fitting on the wall high level presents additional risks when its more than possible that an extension lead and pistol drill (with universal brushed motor) might be dragged down there to drill out rusted bolts, etc.

I'm not saying the rules regarding this are to be ignored, I'm just interested in what they actually require in a situation like this

 

[ban-all-sheds]

Pits in garages
- will fill with water if not properly sealed, and properly sealing them is virtually impossible

The one in my Dad's garage never did.

- will fill with leaking petrol / exhaust fumes / other gases

Ditto, but I can see how that could be a nasty invisible hazard.

- are a potentially explosive environment, which makes the installation of lights and other electrical equipment far more expensive, and far beyond the scope of most electricians, never mind DIY.

Again ditto - we never got an explosive environment in ours, at least not one explosive enough to be ignited by a regular inspection lamp or by brazing, or I wouldn't be here.

- are a hazard as people / cars / pets / other can fall into them

Just *******-well pay attention not to leave it uncovered.

 

[aaronjb]

Oh - back on the subject of SWA cable

Two questions:

1) The electrician I had out originally suggested 6mm SWA to the garage to support a 32A ring and 6A lighting circuit - that seemed small to me based on the rated capacities of the cable, but I'm aware there are all sorts of tables to use to calculate these things.. I'd like to have enough capacity to run a commando socket and welder in there, so - 6mm or 10mm?

2) What size conduit for the SWA? I saw 50mm suggested in another thread, and I gather solid rather than flexible is the best idea (i.e. not Kopex)?

OK I lied, 3 questions:

3) The conduit for the pit - as that's going under/in a concrete floor, does the cable need to be SWA or is T&E enough through PVC conduit? It's not like someone is going to be digging through the garage floor with the power still on..

Just trying to understand the job so I can get all the necessary conduit bits in place before the groundworks people come to lay the slab

 

[RF Lighting]

Personally 6.0mm² seems a bit small to me. I've not seen how long the run from the supply position to the garage is, but IMO you would be better off putting in a bigger cable than you require now, rather than finding out a few years down the line that it isn't big enough to supply the super duper new machine you want to buy, and having to get it replaced.


You initially say you want a socket circuit, some lights and a dedicated welder socket. Already that's pretty close to the limit for 6.0mm² cable, and then maybe add a heater in winter, and a car battery booster and....

I've got a bit more stuff than that in my garage, and another small submains running off it, but I ran a 16mm² supply out to mine. I'd certainly want to put in 10mm² or even 16mm² to your garage if you can afford it.


This is the best duct to use

http://www.drainageonline.co.uk/Ducting/Black-Twinwall-Duct-x-50m-coil.htm

Get the 90mm stuff and you'll be fine. You can use some of this from the garage fuse box to the pit too.

Make sure you bung the ends up when you put it in to stop it getting accidentally filled with concrete as the slab is poured.

Also think obout running another duct in for things like a telephone, burglar alarm, computer network etc that you may want in the future.

Even if you don't use it now, it'll be a God send when you do want something like that.

You should still use SWA for the supply to your pit. It won't need to be big cable, and won't cost a great deal.

 

[aaronjb]

Thanks for the link - much appreciated

6mm seemed small to me, too, as a lay person - then again, said electrician seemed intent on convincing me that in order to add the feed to the garage he'd need to rewire most of the house... (I should add - my house CU is a 1970's metal boxed plug-in breaker type)

I don't mind paying a bit extra to wire in something future proof so the cost difference for 10 or 16mm isn't so great, the run is only around 5m from house CU to (proposed) garage wall so we're talking £60 of cable

Ditto for the pit, it's only 5m or so from where I envisage having the CU to the pit edge.

Heck, even if I put the CU on the far wall of the garage it'd be a 10m run from house to there and 6-7m to the pit edge.

I guess that 90mm duct is going to have to be sunk into the hardcore rather than the concrete, mind - the concrete is only 150mm thick


One last daft question (I think ) - would you run the duct up the side of the house wall and then put the ducting up to the wall (bend radius would be a bit of an issue I guess) or terminate the duct at ground level with a cover of some form to prevent it filling with water?

 

[ajrobb]

My amateur take: 6mm² SWA for 10m underground should support 40A with a 1.2% voltage drop (good enough for lighting and power). This should be enough for a 32A circuit for 13A sockets and a 6A lighting circuit. The socket radial circuit can be wired in 4mm² T+E if clipped direct or buried in plaster (or render?). In a garage you might prefer to use MICC or earthed steel conduit on the surface. You can also run additional circuits within the garage for different socket types. I don't think it is essential that the underground cable can support the full load of all the circuits. It is up to you and your electrician to determine the maximum load.

I'd guess pit lighting poses a special explosion risk as there can be a risk of petrol sitting in the pit with heavy vapour not escaping. The light can be turned on from outside the pit before the spill is cleaned up. For the same reasons, I'd be wary of a fan with an open commutator causing sparks.

 

[aaronjb]

Oops, missed your reply there originally

Thanks! I think I'll probably end up going with 10mm SWA, just in case and for future expansion (granted I couldn't pull too much or I'd just fuse the main 100A house fuse ) given the cost difference.

Then yep, a 32A circuit for sockets and 6A lighting ideally run in earthed steel conduit .. although that might add quite a bit to the cost, so the first fit might just end up as 4mm T&E clipped direct - at least until the garage ends up getting panneled out. That might take a while!

 

[securespark]

You could use anticorrosives in the pit. Then they have a reasonable amount of impact protection.